It depends -
Since you only have a Zero (0) in box 12a of your W-2, when you are entering your W-2 into TurboTax, just leave box 12a blank (no entry).
If you have an amount on line 12a but no code, then you will need to contact your employer to request this information and possibly a correct W-2.
If you are still having issues with your W-2 after changing box 12a from zero to blank (no entry) then you may want to delete your W-2 all together and reenter with the 12a section left blank.